1. Field of the Invention
This invention relates to the field of loading liquefied gases into containers.
2. Description of Related Art
Typically a filling station has a large storage tank in which a cryogenic substance is stored in liquid form. Portable cylinders, which are superinsulated to maintain the cryogenic substance in its liquid form, must be periodically refilled from these filling stations and transported to a place of use.
During the transfer of liquefied gases from the storage tank to the portable cylinder, a portion of the product gas is wasted. These filling losses, depending on the circumstances, may be a significant percentage of the product gas.
It is known to transfer cryogenic substances from a storage tank to a liquid cylinder using pressurized transfer filling and centrifugal pump filling. In pressurized transfer filling the pressure head within the storage tank is used to force substance through pipes into a cylinder. In centrifugal pump filling, a centrifugal pump is disposed in line between the storage tank and the liquid cylinder for transferring substance.
The cylinder being filled includes two connections associated with filling, an inlet port and an outlet vent. Substance is loaded into the cylinder through the inlet port while the outlet vent is left open, allowing any liquefied gas which returns to a gaseous form to vent to the atmosphere. As substance flows through a filling station the substance absorbs heat causing the substance to change state into gas and causing high venting losses due to excessive flashing from the pressure letdown between storage tank and cylinder pressure as a substance enters the cylinder.
The conventional liquid cylinder filling technique fills the cylinder with the vent valve completely open, incurring high venting losses due to excessive flashing of the cryogenic entering the cylinder. Fill line length, diameter, restrictions, and insulation potential contribute to the overall filling loss. Many liquid cylinders are overfilled due to operator error. This can present a serious safety hazard in those fill plants where the cylinder vents into the enclosed fill building. This liquid fill venting mode also causes the filling loss to drastically increase and results in a cylinder which is filled beyond the DOT fill density limit, requiring further venting to achieve the shipping limit.
A number of prior systems have attempted to deal with these large filling losses. These systems include recirculating systems to prevent loss of flashed vapor, top filling the cylinder with pumps and pump aided transfer systems. None of these have been entirely satisfactory.
The recirculating systems have recirculated the flashed vapor generated when the liquid from the tank has entered the cylinder. Recirculating the flashed vapor back to the tank can result in a no loss system. However, there is a serious risk of contamination of the tank if a contaminated liquid cylinder has been filled. Also the heat absorbed by the recirculated vapor is added to the storage tank, an undesirable event. Further, a sophisticated operator is required to run this system.
Top filling with a pump generally has operated only under ideal conditions in which the plumbing between tank and cylinder is precooled and the liquid cylinder is cold. Under typical conditions the cylinder must be blown down periodically to avoid losing pump prime or damaging the seals. Further, the operation takes 10 to 12 minutes on average and requires a sophisticated operator to deal with pump problems and maintenance.
U.S. Pat. No. 4,475,348 discloses use of an automatic throttling valve to vent the cylinder being filled whenever the pressure in the cylinder reaches approximately 10 PSI less than the pressure in the storage tank regardless of filling station configuration or substance. This method decreases filling losses to some degree but its effectiveness varies with station configuration and substance.
It is known that during centrifugal pump transfer of substance from a storage tank to a cylinder, centrifugal pumps are subject to cavitation. Cavitation is caused when the cryogenic substance absorbs thermal energy causing the substance to vaporize in the pump inlet and bubbles of the vapor to be carried to the impeller of the pump. The pump rotor then spins more rapidly in the gas bubble since the gas offers much less resistance than the liquid. This rapid spinning causes friction and heat which warms the gas further causing further vaporization. Unless the motor is stopped when this occurs, the pump motor could burn out or the casing or rotor of the motor could break due to internal friction. If the substance being loaded is liquid oxygen, there is a high potential for a safety hazard.
Rattan in "Cryogenic Liquid Service", Chemical Engineering, Apr. 1, 1985, page 95 discloses bleeding a small liquid stream through a hole in a pump to keep the pump cool to deal with this problem. However, in very hot areas, a large amount of substance must be wasted by this method. Another method disclosed in this same article, is bringing the pressure within a system up to a level that prevents flashing.
Another danger present when liquid cylinders are loaded with a cryogenic substance is that if the cylinder is overfilled, liquefied gas product is discharged from the outlet vent of the cylinder. It was a common practice to continue filling a cylinder until liquefied product was discharged from the outlet vent as a way of determining when the cylinder was full. In addition to wasting product this can be dangerous since the liquefied gas may injure an operator by cryogenic burns or asphyxiation or cause an explosion or a fire.